Commit | Line | Data |
---|---|---|
3cb98950 DH |
1 | ======================================== |
2 | GENERIC ASSOCIATIVE ARRAY IMPLEMENTATION | |
3 | ======================================== | |
4 | ||
5 | Contents: | |
6 | ||
7 | - Overview. | |
8 | ||
9 | - The public API. | |
10 | - Edit script. | |
11 | - Operations table. | |
12 | - Manipulation functions. | |
13 | - Access functions. | |
14 | - Index key form. | |
15 | ||
16 | - Internal workings. | |
17 | - Basic internal tree layout. | |
18 | - Shortcuts. | |
19 | - Splitting and collapsing nodes. | |
20 | - Non-recursive iteration. | |
21 | - Simultaneous alteration and iteration. | |
22 | ||
23 | ||
24 | ======== | |
25 | OVERVIEW | |
26 | ======== | |
27 | ||
28 | This associative array implementation is an object container with the following | |
29 | properties: | |
30 | ||
31 | (1) Objects are opaque pointers. The implementation does not care where they | |
32 | point (if anywhere) or what they point to (if anything). | |
33 | ||
34 | [!] NOTE: Pointers to objects _must_ be zero in the least significant bit. | |
35 | ||
36 | (2) Objects do not need to contain linkage blocks for use by the array. This | |
37 | permits an object to be located in multiple arrays simultaneously. | |
38 | Rather, the array is made up of metadata blocks that point to objects. | |
39 | ||
40 | (3) Objects require index keys to locate them within the array. | |
41 | ||
42 | (4) Index keys must be unique. Inserting an object with the same key as one | |
43 | already in the array will replace the old object. | |
44 | ||
45 | (5) Index keys can be of any length and can be of different lengths. | |
46 | ||
47 | (6) Index keys should encode the length early on, before any variation due to | |
48 | length is seen. | |
49 | ||
50 | (7) Index keys can include a hash to scatter objects throughout the array. | |
51 | ||
52 | (8) The array can iterated over. The objects will not necessarily come out in | |
53 | key order. | |
54 | ||
55 | (9) The array can be iterated over whilst it is being modified, provided the | |
56 | RCU readlock is being held by the iterator. Note, however, under these | |
57 | circumstances, some objects may be seen more than once. If this is a | |
58 | problem, the iterator should lock against modification. Objects will not | |
59 | be missed, however, unless deleted. | |
60 | ||
61 | (10) Objects in the array can be looked up by means of their index key. | |
62 | ||
63 | (11) Objects can be looked up whilst the array is being modified, provided the | |
64 | RCU readlock is being held by the thread doing the look up. | |
65 | ||
66 | The implementation uses a tree of 16-pointer nodes internally that are indexed | |
67 | on each level by nibbles from the index key in the same manner as in a radix | |
68 | tree. To improve memory efficiency, shortcuts can be emplaced to skip over | |
69 | what would otherwise be a series of single-occupancy nodes. Further, nodes | |
70 | pack leaf object pointers into spare space in the node rather than making an | |
71 | extra branch until as such time an object needs to be added to a full node. | |
72 | ||
73 | ||
74 | ============== | |
75 | THE PUBLIC API | |
76 | ============== | |
77 | ||
78 | The public API can be found in <linux/assoc_array.h>. The associative array is | |
79 | rooted on the following structure: | |
80 | ||
81 | struct assoc_array { | |
82 | ... | |
83 | }; | |
84 | ||
85 | The code is selected by enabling CONFIG_ASSOCIATIVE_ARRAY. | |
86 | ||
87 | ||
88 | EDIT SCRIPT | |
89 | ----------- | |
90 | ||
91 | The insertion and deletion functions produce an 'edit script' that can later be | |
92 | applied to effect the changes without risking ENOMEM. This retains the | |
93 | preallocated metadata blocks that will be installed in the internal tree and | |
94 | keeps track of the metadata blocks that will be removed from the tree when the | |
95 | script is applied. | |
96 | ||
97 | This is also used to keep track of dead blocks and dead objects after the | |
98 | script has been applied so that they can be freed later. The freeing is done | |
99 | after an RCU grace period has passed - thus allowing access functions to | |
100 | proceed under the RCU read lock. | |
101 | ||
102 | The script appears as outside of the API as a pointer of the type: | |
103 | ||
104 | struct assoc_array_edit; | |
105 | ||
106 | There are two functions for dealing with the script: | |
107 | ||
108 | (1) Apply an edit script. | |
109 | ||
110 | void assoc_array_apply_edit(struct assoc_array_edit *edit); | |
111 | ||
112 | This will perform the edit functions, interpolating various write barriers | |
113 | to permit accesses under the RCU read lock to continue. The edit script | |
114 | will then be passed to call_rcu() to free it and any dead stuff it points | |
115 | to. | |
116 | ||
117 | (2) Cancel an edit script. | |
118 | ||
119 | void assoc_array_cancel_edit(struct assoc_array_edit *edit); | |
120 | ||
121 | This frees the edit script and all preallocated memory immediately. If | |
122 | this was for insertion, the new object is _not_ released by this function, | |
123 | but must rather be released by the caller. | |
124 | ||
125 | These functions are guaranteed not to fail. | |
126 | ||
127 | ||
128 | OPERATIONS TABLE | |
129 | ---------------- | |
130 | ||
131 | Various functions take a table of operations: | |
132 | ||
133 | struct assoc_array_ops { | |
134 | ... | |
135 | }; | |
136 | ||
137 | This points to a number of methods, all of which need to be provided: | |
138 | ||
139 | (1) Get a chunk of index key from caller data: | |
140 | ||
141 | unsigned long (*get_key_chunk)(const void *index_key, int level); | |
142 | ||
143 | This should return a chunk of caller-supplied index key starting at the | |
144 | *bit* position given by the level argument. The level argument will be a | |
145 | multiple of ASSOC_ARRAY_KEY_CHUNK_SIZE and the function should return | |
146 | ASSOC_ARRAY_KEY_CHUNK_SIZE bits. No error is possible. | |
147 | ||
148 | ||
149 | (2) Get a chunk of an object's index key. | |
150 | ||
151 | unsigned long (*get_object_key_chunk)(const void *object, int level); | |
152 | ||
153 | As the previous function, but gets its data from an object in the array | |
154 | rather than from a caller-supplied index key. | |
155 | ||
156 | ||
157 | (3) See if this is the object we're looking for. | |
158 | ||
159 | bool (*compare_object)(const void *object, const void *index_key); | |
160 | ||
161 | Compare the object against an index key and return true if it matches and | |
162 | false if it doesn't. | |
163 | ||
164 | ||
165 | (4) Diff the index keys of two objects. | |
166 | ||
23fd78d7 | 167 | int (*diff_objects)(const void *object, const void *index_key); |
3cb98950 | 168 | |
23fd78d7 DH |
169 | Return the bit position at which the index key of the specified object |
170 | differs from the given index key or -1 if they are the same. | |
3cb98950 DH |
171 | |
172 | ||
173 | (5) Free an object. | |
174 | ||
175 | void (*free_object)(void *object); | |
176 | ||
177 | Free the specified object. Note that this may be called an RCU grace | |
178 | period after assoc_array_apply_edit() was called, so synchronize_rcu() may | |
179 | be necessary on module unloading. | |
180 | ||
181 | ||
182 | MANIPULATION FUNCTIONS | |
183 | ---------------------- | |
184 | ||
185 | There are a number of functions for manipulating an associative array: | |
186 | ||
187 | (1) Initialise an associative array. | |
188 | ||
189 | void assoc_array_init(struct assoc_array *array); | |
190 | ||
191 | This initialises the base structure for an associative array. It can't | |
192 | fail. | |
193 | ||
194 | ||
195 | (2) Insert/replace an object in an associative array. | |
196 | ||
197 | struct assoc_array_edit * | |
198 | assoc_array_insert(struct assoc_array *array, | |
199 | const struct assoc_array_ops *ops, | |
200 | const void *index_key, | |
201 | void *object); | |
202 | ||
203 | This inserts the given object into the array. Note that the least | |
204 | significant bit of the pointer must be zero as it's used to type-mark | |
205 | pointers internally. | |
206 | ||
207 | If an object already exists for that key then it will be replaced with the | |
208 | new object and the old one will be freed automatically. | |
209 | ||
210 | The index_key argument should hold index key information and is | |
211 | passed to the methods in the ops table when they are called. | |
212 | ||
213 | This function makes no alteration to the array itself, but rather returns | |
214 | an edit script that must be applied. -ENOMEM is returned in the case of | |
215 | an out-of-memory error. | |
216 | ||
217 | The caller should lock exclusively against other modifiers of the array. | |
218 | ||
219 | ||
220 | (3) Delete an object from an associative array. | |
221 | ||
222 | struct assoc_array_edit * | |
223 | assoc_array_delete(struct assoc_array *array, | |
224 | const struct assoc_array_ops *ops, | |
225 | const void *index_key); | |
226 | ||
227 | This deletes an object that matches the specified data from the array. | |
228 | ||
229 | The index_key argument should hold index key information and is | |
230 | passed to the methods in the ops table when they are called. | |
231 | ||
232 | This function makes no alteration to the array itself, but rather returns | |
233 | an edit script that must be applied. -ENOMEM is returned in the case of | |
234 | an out-of-memory error. NULL will be returned if the specified object is | |
235 | not found within the array. | |
236 | ||
237 | The caller should lock exclusively against other modifiers of the array. | |
238 | ||
239 | ||
240 | (4) Delete all objects from an associative array. | |
241 | ||
242 | struct assoc_array_edit * | |
243 | assoc_array_clear(struct assoc_array *array, | |
244 | const struct assoc_array_ops *ops); | |
245 | ||
246 | This deletes all the objects from an associative array and leaves it | |
247 | completely empty. | |
248 | ||
249 | This function makes no alteration to the array itself, but rather returns | |
250 | an edit script that must be applied. -ENOMEM is returned in the case of | |
251 | an out-of-memory error. | |
252 | ||
253 | The caller should lock exclusively against other modifiers of the array. | |
254 | ||
255 | ||
256 | (5) Destroy an associative array, deleting all objects. | |
257 | ||
258 | void assoc_array_destroy(struct assoc_array *array, | |
259 | const struct assoc_array_ops *ops); | |
260 | ||
261 | This destroys the contents of the associative array and leaves it | |
262 | completely empty. It is not permitted for another thread to be traversing | |
263 | the array under the RCU read lock at the same time as this function is | |
264 | destroying it as no RCU deferral is performed on memory release - | |
265 | something that would require memory to be allocated. | |
266 | ||
267 | The caller should lock exclusively against other modifiers and accessors | |
268 | of the array. | |
269 | ||
270 | ||
271 | (6) Garbage collect an associative array. | |
272 | ||
273 | int assoc_array_gc(struct assoc_array *array, | |
274 | const struct assoc_array_ops *ops, | |
275 | bool (*iterator)(void *object, void *iterator_data), | |
276 | void *iterator_data); | |
277 | ||
278 | This iterates over the objects in an associative array and passes each one | |
279 | to iterator(). If iterator() returns true, the object is kept. If it | |
280 | returns false, the object will be freed. If the iterator() function | |
281 | returns true, it must perform any appropriate refcount incrementing on the | |
282 | object before returning. | |
283 | ||
284 | The internal tree will be packed down if possible as part of the iteration | |
285 | to reduce the number of nodes in it. | |
286 | ||
287 | The iterator_data is passed directly to iterator() and is otherwise | |
288 | ignored by the function. | |
289 | ||
290 | The function will return 0 if successful and -ENOMEM if there wasn't | |
291 | enough memory. | |
292 | ||
293 | It is possible for other threads to iterate over or search the array under | |
294 | the RCU read lock whilst this function is in progress. The caller should | |
295 | lock exclusively against other modifiers of the array. | |
296 | ||
297 | ||
298 | ACCESS FUNCTIONS | |
299 | ---------------- | |
300 | ||
301 | There are two functions for accessing an associative array: | |
302 | ||
303 | (1) Iterate over all the objects in an associative array. | |
304 | ||
305 | int assoc_array_iterate(const struct assoc_array *array, | |
306 | int (*iterator)(const void *object, | |
307 | void *iterator_data), | |
308 | void *iterator_data); | |
309 | ||
310 | This passes each object in the array to the iterator callback function. | |
311 | iterator_data is private data for that function. | |
312 | ||
313 | This may be used on an array at the same time as the array is being | |
314 | modified, provided the RCU read lock is held. Under such circumstances, | |
315 | it is possible for the iteration function to see some objects twice. If | |
316 | this is a problem, then modification should be locked against. The | |
317 | iteration algorithm should not, however, miss any objects. | |
318 | ||
319 | The function will return 0 if no objects were in the array or else it will | |
320 | return the result of the last iterator function called. Iteration stops | |
321 | immediately if any call to the iteration function results in a non-zero | |
322 | return. | |
323 | ||
324 | ||
325 | (2) Find an object in an associative array. | |
326 | ||
327 | void *assoc_array_find(const struct assoc_array *array, | |
328 | const struct assoc_array_ops *ops, | |
329 | const void *index_key); | |
330 | ||
331 | This walks through the array's internal tree directly to the object | |
332 | specified by the index key.. | |
333 | ||
334 | This may be used on an array at the same time as the array is being | |
335 | modified, provided the RCU read lock is held. | |
336 | ||
337 | The function will return the object if found (and set *_type to the object | |
338 | type) or will return NULL if the object was not found. | |
339 | ||
340 | ||
341 | INDEX KEY FORM | |
342 | -------------- | |
343 | ||
344 | The index key can be of any form, but since the algorithms aren't told how long | |
345 | the key is, it is strongly recommended that the index key includes its length | |
346 | very early on before any variation due to the length would have an effect on | |
347 | comparisons. | |
348 | ||
349 | This will cause leaves with different length keys to scatter away from each | |
350 | other - and those with the same length keys to cluster together. | |
351 | ||
352 | It is also recommended that the index key begin with a hash of the rest of the | |
353 | key to maximise scattering throughout keyspace. | |
354 | ||
355 | The better the scattering, the wider and lower the internal tree will be. | |
356 | ||
357 | Poor scattering isn't too much of a problem as there are shortcuts and nodes | |
358 | can contain mixtures of leaves and metadata pointers. | |
359 | ||
360 | The index key is read in chunks of machine word. Each chunk is subdivided into | |
361 | one nibble (4 bits) per level, so on a 32-bit CPU this is good for 8 levels and | |
362 | on a 64-bit CPU, 16 levels. Unless the scattering is really poor, it is | |
363 | unlikely that more than one word of any particular index key will have to be | |
364 | used. | |
365 | ||
366 | ||
367 | ================= | |
368 | INTERNAL WORKINGS | |
369 | ================= | |
370 | ||
371 | The associative array data structure has an internal tree. This tree is | |
372 | constructed of two types of metadata blocks: nodes and shortcuts. | |
373 | ||
374 | A node is an array of slots. Each slot can contain one of four things: | |
375 | ||
376 | (*) A NULL pointer, indicating that the slot is empty. | |
377 | ||
378 | (*) A pointer to an object (a leaf). | |
379 | ||
380 | (*) A pointer to a node at the next level. | |
381 | ||
382 | (*) A pointer to a shortcut. | |
383 | ||
384 | ||
385 | BASIC INTERNAL TREE LAYOUT | |
386 | -------------------------- | |
387 | ||
388 | Ignoring shortcuts for the moment, the nodes form a multilevel tree. The index | |
389 | key space is strictly subdivided by the nodes in the tree and nodes occur on | |
390 | fixed levels. For example: | |
391 | ||
392 | Level: 0 1 2 3 | |
393 | =============== =============== =============== =============== | |
394 | NODE D | |
395 | NODE B NODE C +------>+---+ | |
396 | +------>+---+ +------>+---+ | | 0 | | |
397 | NODE A | | 0 | | | 0 | | +---+ | |
398 | +---+ | +---+ | +---+ | : : | |
399 | | 0 | | : : | : : | +---+ | |
400 | +---+ | +---+ | +---+ | | f | | |
401 | | 1 |---+ | 3 |---+ | 7 |---+ +---+ | |
402 | +---+ +---+ +---+ | |
403 | : : : : | 8 |---+ | |
404 | +---+ +---+ +---+ | NODE E | |
405 | | e |---+ | f | : : +------>+---+ | |
406 | +---+ | +---+ +---+ | 0 | | |
407 | | f | | | f | +---+ | |
408 | +---+ | +---+ : : | |
409 | | NODE F +---+ | |
410 | +------>+---+ | f | | |
411 | | 0 | NODE G +---+ | |
412 | +---+ +------>+---+ | |
413 | : : | | 0 | | |
414 | +---+ | +---+ | |
415 | | 6 |---+ : : | |
416 | +---+ +---+ | |
417 | : : | f | | |
418 | +---+ +---+ | |
419 | | f | | |
420 | +---+ | |
421 | ||
422 | In the above example, there are 7 nodes (A-G), each with 16 slots (0-f). | |
423 | Assuming no other meta data nodes in the tree, the key space is divided thusly: | |
424 | ||
425 | KEY PREFIX NODE | |
426 | ========== ==== | |
427 | 137* D | |
428 | 138* E | |
429 | 13[0-69-f]* C | |
430 | 1[0-24-f]* B | |
431 | e6* G | |
432 | e[0-57-f]* F | |
433 | [02-df]* A | |
434 | ||
435 | So, for instance, keys with the following example index keys will be found in | |
436 | the appropriate nodes: | |
437 | ||
438 | INDEX KEY PREFIX NODE | |
439 | =============== ======= ==== | |
440 | 13694892892489 13 C | |
441 | 13795289025897 137 D | |
442 | 13889dde88793 138 E | |
443 | 138bbb89003093 138 E | |
444 | 1394879524789 12 C | |
445 | 1458952489 1 B | |
446 | 9431809de993ba - A | |
447 | b4542910809cd - A | |
448 | e5284310def98 e F | |
449 | e68428974237 e6 G | |
450 | e7fffcbd443 e F | |
451 | f3842239082 - A | |
452 | ||
453 | To save memory, if a node can hold all the leaves in its portion of keyspace, | |
454 | then the node will have all those leaves in it and will not have any metadata | |
455 | pointers - even if some of those leaves would like to be in the same slot. | |
456 | ||
457 | A node can contain a heterogeneous mix of leaves and metadata pointers. | |
458 | Metadata pointers must be in the slots that match their subdivisions of key | |
459 | space. The leaves can be in any slot not occupied by a metadata pointer. It | |
460 | is guaranteed that none of the leaves in a node will match a slot occupied by a | |
461 | metadata pointer. If the metadata pointer is there, any leaf whose key matches | |
462 | the metadata key prefix must be in the subtree that the metadata pointer points | |
463 | to. | |
464 | ||
465 | In the above example list of index keys, node A will contain: | |
466 | ||
467 | SLOT CONTENT INDEX KEY (PREFIX) | |
468 | ==== =============== ================== | |
469 | 1 PTR TO NODE B 1* | |
470 | any LEAF 9431809de993ba | |
471 | any LEAF b4542910809cd | |
472 | e PTR TO NODE F e* | |
473 | any LEAF f3842239082 | |
474 | ||
475 | and node B: | |
476 | ||
477 | 3 PTR TO NODE C 13* | |
478 | any LEAF 1458952489 | |
479 | ||
480 | ||
481 | SHORTCUTS | |
482 | --------- | |
483 | ||
484 | Shortcuts are metadata records that jump over a piece of keyspace. A shortcut | |
485 | is a replacement for a series of single-occupancy nodes ascending through the | |
486 | levels. Shortcuts exist to save memory and to speed up traversal. | |
487 | ||
488 | It is possible for the root of the tree to be a shortcut - say, for example, | |
489 | the tree contains at least 17 nodes all with key prefix '1111'. The insertion | |
490 | algorithm will insert a shortcut to skip over the '1111' keyspace in a single | |
491 | bound and get to the fourth level where these actually become different. | |
492 | ||
493 | ||
494 | SPLITTING AND COLLAPSING NODES | |
495 | ------------------------------ | |
496 | ||
497 | Each node has a maximum capacity of 16 leaves and metadata pointers. If the | |
498 | insertion algorithm finds that it is trying to insert a 17th object into a | |
499 | node, that node will be split such that at least two leaves that have a common | |
500 | key segment at that level end up in a separate node rooted on that slot for | |
501 | that common key segment. | |
502 | ||
503 | If the leaves in a full node and the leaf that is being inserted are | |
504 | sufficiently similar, then a shortcut will be inserted into the tree. | |
505 | ||
506 | When the number of objects in the subtree rooted at a node falls to 16 or | |
507 | fewer, then the subtree will be collapsed down to a single node - and this will | |
508 | ripple towards the root if possible. | |
509 | ||
510 | ||
511 | NON-RECURSIVE ITERATION | |
512 | ----------------------- | |
513 | ||
514 | Each node and shortcut contains a back pointer to its parent and the number of | |
515 | slot in that parent that points to it. None-recursive iteration uses these to | |
516 | proceed rootwards through the tree, going to the parent node, slot N + 1 to | |
517 | make sure progress is made without the need for a stack. | |
518 | ||
519 | The backpointers, however, make simultaneous alteration and iteration tricky. | |
520 | ||
521 | ||
522 | SIMULTANEOUS ALTERATION AND ITERATION | |
523 | ------------------------------------- | |
524 | ||
525 | There are a number of cases to consider: | |
526 | ||
527 | (1) Simple insert/replace. This involves simply replacing a NULL or old | |
528 | matching leaf pointer with the pointer to the new leaf after a barrier. | |
529 | The metadata blocks don't change otherwise. An old leaf won't be freed | |
530 | until after the RCU grace period. | |
531 | ||
532 | (2) Simple delete. This involves just clearing an old matching leaf. The | |
533 | metadata blocks don't change otherwise. The old leaf won't be freed until | |
534 | after the RCU grace period. | |
535 | ||
536 | (3) Insertion replacing part of a subtree that we haven't yet entered. This | |
537 | may involve replacement of part of that subtree - but that won't affect | |
538 | the iteration as we won't have reached the pointer to it yet and the | |
539 | ancestry blocks are not replaced (the layout of those does not change). | |
540 | ||
541 | (4) Insertion replacing nodes that we're actively processing. This isn't a | |
542 | problem as we've passed the anchoring pointer and won't switch onto the | |
543 | new layout until we follow the back pointers - at which point we've | |
544 | already examined the leaves in the replaced node (we iterate over all the | |
545 | leaves in a node before following any of its metadata pointers). | |
546 | ||
547 | We might, however, re-see some leaves that have been split out into a new | |
548 | branch that's in a slot further along than we were at. | |
549 | ||
550 | (5) Insertion replacing nodes that we're processing a dependent branch of. | |
551 | This won't affect us until we follow the back pointers. Similar to (4). | |
552 | ||
553 | (6) Deletion collapsing a branch under us. This doesn't affect us because the | |
554 | back pointers will get us back to the parent of the new node before we | |
555 | could see the new node. The entire collapsed subtree is thrown away | |
556 | unchanged - and will still be rooted on the same slot, so we shouldn't | |
557 | process it a second time as we'll go back to slot + 1. | |
558 | ||
559 | Note: | |
560 | ||
561 | (*) Under some circumstances, we need to simultaneously change the parent | |
562 | pointer and the parent slot pointer on a node (say, for example, we | |
563 | inserted another node before it and moved it up a level). We cannot do | |
564 | this without locking against a read - so we have to replace that node too. | |
565 | ||
566 | However, when we're changing a shortcut into a node this isn't a problem | |
567 | as shortcuts only have one slot and so the parent slot number isn't used | |
568 | when traversing backwards over one. This means that it's okay to change | |
569 | the slot number first - provided suitable barriers are used to make sure | |
570 | the parent slot number is read after the back pointer. | |
571 | ||
572 | Obsolete blocks and leaves are freed up after an RCU grace period has passed, | |
573 | so as long as anyone doing walking or iteration holds the RCU read lock, the | |
574 | old superstructure should not go away on them. |